Subject: Re: [PATCH v2] powerpc/numa: set node_possible_map to only node_online_map during boot

From: Raghavendra K T <[email-masked]>
Date: Fri, 06 Mar 2015 16:59:45 +0530

On 03/06/2015 10:57 AM, Nishanth Aravamudan wrote:
> On 05.03.2015 [15:29:00 -0800], David Rientjes wrote:
>> On Thu, 5 Mar 2015, Nishanth Aravamudan wrote:
>>
>>> So if we compare to x86:
>>>
>>> arch/x86/mm/numa.c::numa_init():
>>>
>>>          nodes_clear(numa_nodes_parsed);
>>>          nodes_clear(node_possible_map);
>>>          nodes_clear(node_online_map);
>>> 	...
>>> 	numa_register_memblks(...);
>>>
>>> arch/x86/mm/numa.c::numa_register_memblks():
>>>
>>> 	node_possible_map = numa_nodes_parsed;
>>>
>>> Basically, it looks like x86 NUMA init clears out possible map and
>>> online map, probably for a similar reason to what I gave in the
>>> changelog that by default, the possible map seems to be based off
>>> MAX_NUMNODES, rather than nr_node_ids or anything dynamic.
>>>
>>> My patch was an attempt to emulate the same thing on powerpc. You are
>>> right that there is a window in which the node_possible_map and
>>> node_online_map are out of sync with my patch. It seems like it
>>> shouldn't matter given how early in boot we are, but perhaps the
>>> following would have been clearer:
>>>
>>> diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
>>> index 0257a7d659ef..1a118b08fad2 100644
>>> --- a/arch/powerpc/mm/numa.c
>>> +++ b/arch/powerpc/mm/numa.c
>>> @@ -958,6 +958,13 @@ void __init initmem_init(void)
>>>
>>>          memblock_dump_all();
>>>
>>> +       /*
>>> +        * Reduce the possible NUMA nodes to the online NUMA nodes,
>>> +        * since we do not support node hotplug. This ensures that  we
>>> +        * lower the maximum NUMA node ID to what is actually present.
>>> +        */
>>> +       nodes_and(node_possible_map, node_possible_map, node_online_map);
>>
>> If you don't support node hotplug, then a node should always be possible
>> if it's online unless there are other tricks powerpc plays with
>> node_possible_map.  Shouldn't this just be
>> node_possible_map = node_online_map?
>
> Yeah, but I was too dumb to think of that before sending :)
>
> Updated version follows...
>
> -Nish
>
---8<---
>
> Raghu noticed an issue with excessive memory allocation on power with a
> simple cgroup test, specifically, in mem_cgroup_css_alloc ->
> for_each_node -> alloc_mem_cgroup_per_zone_info(), which ends up blowing
> up the kmalloc-2048 slab (to the order of 200MB for 400 cgroup
> directories).
should we also add after this patch it has reduced to around 2MB?
>
> The underlying issue is that NODES_SHIFT on power is 8 (256 NUMA nodes
> possible), which defines node_possible_map, which in turn defines the
> value of nr_node_ids in setup_nr_node_ids and the iteration of
> for_each_node.
>
> In practice, we never see a system with 256 NUMA nodes, and in fact, we
> do not support node hotplug on power in the first place, so the nodes
> that are online when we come up are the nodes that will be present for
> the lifetime of this kernel. So let's, at least, drop the NUMA possible
> map down to the online map at runtime. This is similar to what x86 does
> in its initialization routines.
>
> mem_cgroup_css_alloc should also be fixed to only iterate over
> memory-populated nodes and handle hotplug, but that is a separate
> change.
>
Maybe we could fomally add
Reported-by: Raghavendra K T <[email-masked]>
> Signed-off-by: Nishanth Aravamudan <[email-masked]>
> To: Michael Ellerman <[email-masked]>
> Cc: [email-masked]
> Cc: Tejun Heo <[email-masked]>
> Cc: David Rientjes <[email-masked]>
> Cc: Benjamin Herrenschmidt <[email-masked]>
> Cc: Paul Mackerras <[email-masked]>
> Cc: Anton Blanchard <[email-masked]>
> Cc: Raghavendra K T <[email-masked]>
>
> ---
> v1 -> v2:
>    Rather than clear node_possible_map and set it nid-by-nid, just
>    directly assign node_online_map to it, as suggested by Michael
>    Ellerman and Tejun Heo.
>
> diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
> index 0257a7d659ef..0c1716cd271f 100644
> --- a/arch/powerpc/mm/numa.c
> +++ b/arch/powerpc/mm/numa.c
> @@ -958,6 +958,13 @@ void __init initmem_init(void)
>
>   	memblock_dump_all();
>
> +	/*
> +	 * Reduce the possible NUMA nodes to the online NUMA nodes,
> +	 * since we do not support node hotplug. This ensures that  we
> +	 * lower the maximum NUMA node ID to what is actually present.
> +	 */

  Hope we remember this change when we add hotplug :)

> +	node_possible_map = node_online_map;
> +
>   	for_each_online_node(nid) {
>   		unsigned long start_pfn, end_pfn;
>